The present invention relates to porous electrodes for alkaline accumulators which because of their novel structure provide both higher specific capacities and also greater current densities of charging and discharging, in line with the present need for avoiding exhaust gases and noise by changing over to accumulator cars. Even though in principle the invention relates to all kinds of accumulator electrodes, it is nevertheless specially suitable for negatives consisting of porous cobalt in combination with the standard positives on a nickel hydroxide basis or with air-breathing oxygen cathodes in hybrid batteries.
The significance of the porosity for the specific capacity of accumulator electrodes was asserted by various inventors decades ago, but was so inaccurately defined that this parameter could not be distinctly measured nor utilised for the systematic improvement of electrodes. Thus, for example, Thorausch and Schlecht in 1929 in German Pat. 583 869 and Ackermann in 1929 in German Patent 806 122 described processes with which they introduced nickel or iron powder obtained from carbonyl compounds as materials for electrode technology, such as are still usual in industry today. They showed that these powders could be mixed with fillers which could be removed subsequently, they could be sintered without pressure to form porous plates and they gave a number of examples of processes for this purpose. According to Example 1 of Thorausch and Schlecht an electrode sintered from carbonyl iron would have a pore volume of 84% and correlated with this a specific capacity 20% higher than the previously used Edison negatives with a tubular structure. The porous nickel plates sintered in a similar manner by Ackermann were said to possess a pore volume of 76.9%. In none of these basic patents are the size and shape of the pores mentioned.
How little relevant this undefined concept of volume porosity is can be seen clearly from the statement of the inventors that very different numerical values can be found regarding such sintered articles according to the method of measurement employed. The total pore volume can naturally be determined if one (1) weighs the porous electrode and a pore-free article of the same external dimensions and calculates the difference in weight. If as an alternative (2) one uses the so-called BET method of measurement of Brunauer, Emmett and Teller, according to which the porous article is first of all weighed in high vacuum and then in a chemically inert gas atmosphere and the difference in weight resulting from the gas adsorption on the inner surfaces is assessed, the gas cannot penetrate into the closed and so-called "dead" pores, and instead one only determines the internal surface of the open and so-called "transient" pores. But if one 3) investigates the porous article in the mercury porosimeter, the mercury can only penetrate into the wider pores and not into the narrower ones, so that one only determines a part of the transient pores and one obtains a still lower numerical value for the volume porosity than according to 1) and 2).
The observation that the dead porosity can make no contribution to the specific capacity was, it is true, obviously unknown to the earlier inventors mentioned, but apears to be plausible because the electrolyte which on the one hand must supply the OH.sup.- ions for the building up of the metallic hydroxide layer during the discharge of the negative electrode and on the other hand must take it up again during charging has no access to the dead pores.